Automatic test equipment for checking printed circuit boards has long involved use of a "bed of nails" test fixture to which the circuit board is mounted during testing. This test fixture includes a large number of nail-like, spring loaded test probes arranged to make electrical contact under spring pressure with designated test points on the board under test. Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the bed of nails arrangement for contacting test points in a particular board must be customized for that circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and a corresponding array of test probes is configured in the test fixture. This typically involves precision-drilling a pattern of holes in a probe plate to match the customized array of test probes, and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted to the fixture, superimposed over the array of test probes. During testing, the spring loaded test probes are brought into spring pressure contact with the test points on the board under test. Electrical test signals are transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board.
Various approaches have been used in the past for bringing the test probes and the circuit board into pressure contact for in-circuit testing. One class of these fixtures is a wired test fixture in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external test analyzer. These wired test fixtures are often referred to as "vacuum test fixtures" since a vacuum is applied to the interior of the test fixture housing during testing to draw the circuit into contact with the test probes. A movable top plate is mounted over the stationary probe plate and a vacuum seal is formed between the top plate and the probe plate. A second vacuum seal is mounted above the top plate and has a sufficient height to hold the printed circuit board above the spring probes which project through access holes drilled in the top plate for alignment with the underside of the board. During use, a vacuum applied to the region between the probe plate and the top plate is also applied to the underside of the board. This compresses both vacuum seals and pulls the board down against and into electrical contact with the test probes. By maintaining the vacuum seal, the probes are held in spring pressure contact with the test points on the board while the board is tested.
In order for the probes to make contact with the proper test points of the circuit board, the bottom stationary probe plate and the movable top plate which supports the board must remain in a parallel relationship, to hold the board flat while maintaining its position perpendicular to the probe field. A reliable vacuum seal also is necessary.
Various problems have existed in prior art in-circuit test fixtures. U.S. Pat. No. 4,538,104 to Douglas et al. discloses an in-circuit test fixture of the type described above. That patent refers in its background to various technical problems in manufacturing and using such in-circuit test fixtures. One problem has to do with reliably maintaining the alignment parallel to each other and between the top plate and the stationary probe plate, while the test probes are maintained in parallel alignment, perpendicular to the board. The top plate moves toward or away from the stationary probe plate on linear bearings and/or guide pins which allow for alignment of the top plate to the probe field as the board moves up or down. The test probes are disposed in matching patterns of holes drilled in the probe plate and in the top plate. The probes must remain in parallel alignment and the top plate must constantly move in parallel alignment with the probe plate on the bearings without causing any binding between these elements of the test fixture.
The vacuum seal system must prevent vacuum leakage and poor electrical contact between the spring probes and the circuit board under test.
In addition to the need for precisely aligning the top plate and the board with the test probes without mechanical problems, and the need for an efficient vacuum sealing system, the test fixture also should be adapted for easy use under repetitive test conditions by the end-user. The fixture also should be designed so that its component parts can be manufactured and assembled at a reasonable cost.
The present invention provides improvements over the type of in-circuit test fixture shown in the Douglas et al. '104 patent. For instance, the fixture in that patent has thumb screws inside four corners of the fixture that screw into long, narrow, rigid guide shafts affixed to the undersurface of the moving top plate. These long guide shafts fit through the center of linear bearings at the four corners of the probe plate. It can be difficult to remove the top plate because the thumb screws must be accessed from going inside the bottom portion of the fixture to unscrew the thumb screws before the top plate can be lifted out. In addition, the four guide shafts at the corners of the top plate can bind when the user manually removes the top plate. There is a need for an improved system of connecting the moving top plate to the probe plate since it is not desirable for the user to open up the fixture from access in the lower portion of the fixture where much of the complicated wiring is present. However, when the top plate needs to be removed, for service or diagnostic requirements, such as when a test probe must be replaced, it is desirable for the top plate to be removed as easily as possible.
The sealed linear bearing assemblies located at the corners of the Douglas et al. '104 fixture are complex and costly in terms of the number of parts and the labor for assembling the bearings. The present invention provides a greatly improved bearing system.
The Douglas et al. '104 patent also discloses an internal captive seal which surrounds the perimeter of the moving top plate. The top plate seal is a continuous double hollow seal. The underside of the top plate has a peripheral flange which fits down into a deep recess that carries the perimeter seal. The present invention provides a more stable vacuum seal that is manufactured at substantially lower cost. The seal arrangement also allows for "floating" of the top plate, which enables multi-axis freedom of movement of the top plate that is more useful in the proper alignment of the test probe array to test points on a board under test. The seal arrangement of this invention also avoids costly sealed bearings, and therefore substantially reduces manufacturing and labor costs.
There is also a need to provide a linear bearing arrangement that can reliably secure the top plate to the probe plate, without shifting of the top plate relative to the probe field, while also allowing the top plate to be released, when necessary, for shifting in different directions for alignment purposes. Such a bearing support system should be easily usable by the end-user while also being manufactured at a reasonable cost.